Corrosion inhibiting solutions and processes for refrigeration systems comprising halides of a Group Va metallic element

ABSTRACT

Anticorrosion solutions useful for refrigeration processes are disclosed. Halides of Group Va metallic elements can be added to alkali metal halide absorption refrigeration solutions to minimize corrosion of the refrigeration system.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to commonly owned copending ProvisionalApplication Ser. No. 60/022,217, filed Jul. 18, 1996.

FIELD OF THE INVENTION

This invention relates generally to solutions for refrigeration systems,and in particular to absorption solutions which include corrosioninhibiting agents.

BACKGROUND OF THE INVENTION

Absorption refrigeration machines are widely used in commercialoperations. A typical absorption refrigeration machine includes fourmajor sections: absorber, generator, condenser, and evaporator.

In the absorber section, an absorbent fluid, typically an aqueous alkalimetal halide solution, such as a lithium bromide solution, absorbs arefrigerant, typically water vapor. The absorber can be operated undersub-atmospheric pressure. The resultant weak or diluted absorbent fluid(about 40-58% concentration of alkali metal halide rich in therefrigerant) is pumped to the generator. Here heat is applied to thefluid to partially boil off the refrigerant to again concentrate theabsorbent fluid. The concentrated absorbent solution (now about 63-65%alkali metal halide) from the generator is passed through a heatexchanger and then sprayed back into the absorber section where itresumes absorption of the refrigerant vapor.

The refrigerant vapor liberated in the generator migrates to thecondenser where it is liquefied by exchanging heat with a cooling fluid(typically water) flowing through tubing (typically copper based alloytubes) present in the condenser section. The liquid refrigerant in thecondenser moves into the evaporator, which is also operated at anextremely low pressure. In the evaporator, the refrigerant cools therelatively warm system water circulating through the tubes of the tubebundle, and the chilled water is circulated to the load. Heat from thesystem water vaporizes the refrigerant water which then migrates to theabsorber section for absorption into the concentrated solution andcompletes the cycle.

Aqueous alkali metal halide solutions are widely used as absorptionfluids in commercial absorption refrigeration systems. An exemplaryalkali metal halide solution for this application is a lithium bromidesolution, adjusted to pH range of 7-13 with lithium hydroxide. Althoughthis and other types of absorption solutions can be advantageous for therefrigeration cycles, alkali metal halides can be corrosive towards thematerials used to construct the refrigeration machine. Such materialscan include mild and stainless steel for containment components andcopper or copper-nickel alloys for tube bundles, among others.

In addition to the surface damage caused by corrosion, the corrosionreaction evolves hydrogen gas as a byproduct. Incondensibles in the formof atoms or ions can easily enter and diffuse into metals, resulting inthe degradation of their mechanical properties under certain systemconditions.

The severity of corrosion can vary, depending upon factors such astemperature of the system, concentration of alkali metal halide in theabsorption solution, metals used in the construction of therefrigeration unit, the presence of air, and the like. For example,during use, the internal temperatures of such machines can be high,typically up to about 450° F. and higher, depending on the type of theabsorption cycle, which can increase the corrosive effect of the alkalimetal halide solution.

Various additives, such as lithium chromate, lithium nitrate, andlithium molybdate, have been proposed as corrosion inhibitors in alkalimetal halide absorption solutions. However, lithium chromate can raiseenvironmental concerns, and its use is being phased out. Further, thelevel of chromate and its oxidation state must be carefully maintained.If too little chromate is used, then it does not properly passivate thewhole metal surface and pitting can result. Lithium nitrate canpotentially evolve ammonia, which can cause stress corrosion cracking ofcopper based alloys such as heat exchanger tubes. Lithium molybdateexhibits only limited solubility in alkali metal halide solutions. Inaddition, lithium molybdate is metastable in aqueous halide solutionsand thus it can be difficult to maintain a constant concentration ofmolybdate ions in solution.

Antimony oxides have also been proposed as corrosion inhibitors inalkali metal halide absorption solutions. See JP 7-138559 and JP1-74588. However, these compounds also can have limited solubility inalkali metal halide solutions.

SUMMARY OF THE INVENTION

The present invention provides alkali metal halide solutions, preferablylithium halide solutions, and more preferably lithium bromide solutions,which are useful as absorption fluids in refrigeration machines. Theabsorption solutions of the invention contain halides of metallicelements of Group Va of the Periodic Table of Elements, preferablyantimony bromide (SbBr₃), as a corrosion inhibition additive.

The amount of the metallic Group Va element, such as antimony asantimony bromide present in the solution, can vary, depending uponvarious factors. Preferred amounts range from about 50 ppm to about 5000ppm, more preferably about 150 ppm to about 400 ppm.

The resultant solutions can provide comparable and even improvedcorrosion inhibition as compared to solutions which include conventionalcorrosion inhibition additives. For example, the solutions of theinvention can provide good anticorrosion benefits for carbon steel athigh temperatures.

Still further, antimony bromide can exhibit improved solubility inalkali metal halide solutions, such as lithium bromide solutions, ascompared to lithium molybdate and antimony oxide, and accordingly theconcentration of desired anticorrosion ions in solution can beincreased.

Other advantages include reduced corrosion of the refrigeration machine,such as carbon steel, due to vapor phase, reduced amount of hydrogengeneration and enhanced performance of the chillers, due to reducedaccumulation of non-condensable gas in the absorber.

Still further, use of the absorption solutions on a refrigerationmachine can result in a protective layer containing antimony andmagnetite (iron oxide) formed on carbon steel. The inventors have foundthat protective layer which forms as a result of using the solutions ofthe invention can be more corrosion resistant than oxide layers formedin the presence of conventional corrosion inhibitors, such as lithiummolybdate, in which magnetite films tend to be more amorphous and lessdeveloped.

BRIEF DESCRIPTION OF THE DRAWING

Some of the features and advantages of the invention having beendescribed, others will become apparent from the detailed descriptionwhich follows, and from the accompanying drawing, in which:

FIG. 1 is an x-ray diffraction pattern of a surface layer formed oncarbon steel coupons exposed to an antimony bromide (1000 ppm)/LiBr (65wt. %) solution at 400° F. for one week.

DETAILED DESCRIPTION OF THE INVENTION

The compositions of the invention include as a corrosion inhibitingcomponent at least one halide of the Group Va metallic elements (i.e.,arsenic, antimony, and bismuth). The halide can be, for example,bromide, chloride, or iodide and preferably is bromide. Exemplaryhalides of Group Va metallic elements useful as corrosion inhibitingagents in the absorption solutions of the invention include antimonybromide (SbBr₃), arsenic bromide, and bismuth bromide, and the like andmixtures thereof. These Group Va metallic elements as ions in solutioncan exhibit corrosion inhibiting properties in absorption refrigerationsystems. The halides of Group Va metallic elements useful in thesolutions of the invention also are preferably substantially completelysoluble in alkali metal halide solutions, such as lithium bromidesolutions, so as to maximize the concentration of the corrosioninhibiting ions in solution.

The halides of Group Va metallic elements can be used singly or asmixtures with one another and/or with other corrosion inhibiting agents.The halides of Group Va metallic elements are present in the absorptionsolutions in amounts sufficient to provide the desired corrosioninhibiting effect. This amount can vary depending upon various factors,such as the solubility of the halides of Group Va metallic elements inthe absorption solution, the nature of the ions of the halides of GroupVa metallic elements, temperatures of the environment of therefrigeration machine, concentration of the alkali metal halidesolution, metals used in the construction of the refrigeration unit, thepresence of air, and the like. Preferably, the absorption solutions ofthe invention include metallic elements of Group Va as halide in anamount ranging from about 50 parts per million (ppm) to about 5000 ppm,and more preferably from about 150 ppm to about 400 ppm.

The absorption solutions include alkali metal halide in conventionalamounts. An exemplary alkali metal halide solution includes alkali metalhalide in an amount from about 40 to about 65 weight percent, preferablyabout 50 to about 65 weight percent, based on the total weight of thesolution. The alkali metal halide can be, for example, lithium halide,preferably lithium bromide, although the absorption solution can includeother alkali metal halides, such as lithium chloride, lithium iodide, aswell as mixtures of these. Still further, the absorption solution caninclude lithium nitrate. Still further, the absorption solution caninclude other halides, such as zinc halides, which are particularlyuseful in high temperature applications (generally about 450° F. andhigher). The zinc halides can be present in amounts up to about 45weight percent (for example, a solution comprising 45 wt. % zinc halideand 20 wt. % lithium bromide).

As the skilled artisan will appreciate, the environment or conditions,such as temperature and/or pressure, of different refrigeration machinescan vary. Typically, the temperature of the machine ranges from about150° F. to about 500° F., although the temperature can be outside thisrange as well. The solutions of the invention are particularlyadvantageous in higher temperature applications.

The solutions of the invention can include mixtures of halides of GroupVa metallic elements with one another and/or with other corrosioninhibitors as known in the art in conventional amounts.

The present invention will be further illustrated by the followingnon-limiting examples.

EXAMPLE 1

Tests were performed in a pressure vessel autoclave with temperaturecontrol (±2° F.). A concentrated (65 wt. %) lithium bromide solution wasprepared using anhydrous lithium bromide and deionized water. Thealkalinity of each solution was adjusted with lithium hydroxide tocontrol the solution pH at a level that optimized the performance ofeach chemical or generally accepted in the industry.

Sample solutions were prepared by adding antimony bromide or antimonyoxide or lithium molybdate to the lithium bromide solution. Each of theabove inhibitors was added individually to 800 ml lithium bromidesolution. Pre-weighed metal coupons were placed in, and above thelithium bromide solution contained in a 2 liter cylinder made ofInconel-600 material which fits into the autoclave. The cylindercontaining the absorption solution and metal coupons was evacuated (toabout 29.6 in of Hg) using a vacuum pump prior to starting the test. Thesolution was heated to a specified temperature and held at thattemperature for a period of 72 hours.

Upon completion, the test coupons were removed and cleaned. Thecorrosion rate was calculated from the weight loss. The coupons werealso tested by surface analysis. The results are set forth in Table 1below. SbBr₃ exhibits better corrosion protection than that offered byantimony oxide.

                  TABLE 1                                                         ______________________________________                                                 Concen-                                                                             Corrosion Rate (mils per year)                                          tration                                                                             for Carbon Steel at                                            Additive   (ppm)   300° F.                                                                        350° F.                                                                       400° F.                                                                      450° F.                        ______________________________________                                        None               31      41     200   326                                   Antimony bromide                                                              (SbBr.sub.3)                                                                             500     8       3.3    7.0   34                                    pH 11-12   1000    --      --     --    18                                    Antimony oxide                                                                (Sb.sub.2 O.sub.3)                                                                       500     28      24     40    57                                    Lithium Molybdate                                                                        199     28      64     38    43                                    ______________________________________                                    

EXAMPLE 2

Tests were performed as described above in Example 1, except thatcorrosion rates were evaluated for carbon steel coupons in antimonybromide (1000 ppm) in a 65 wt. % LiBr solution at 400° F. for a periodof 168 hours. The corrosion rate was 8.5 mils per year. Hydrogengeneration was also determined to be 2.4 mg/in² of carbon steel. Thecorrosion rate and H₂ evolution in the presence of SbBr₃ is much lowerthan that observed in the presence of 199 ppm of Li₂ MoO₄ (aconventional corrosion inhibitor) at similar conditions (corrosionrate=12 mpy, H₂ =6.8 mg/in² of carbon steel).

The surface of carbon steel coupon treated in the formulation containingantimony bromide was analyzed using X-ray diffraction method. The X-raydiffraction measurements were carried out on a wide angle diffractometerusing CuKα radiation with a diffracted beam monochromator. The carbonsteel metal coupon was exposed to antimony bromide (1000 ppm)/LiBrsolution (65 wt. %) at 400° F. for one week. FIG. 1 presents the X-raydiffraction pattern to a specific area of the metal coupon where theprotective coating was formed after an attack from the brine solution.The peaks seen in the spectra identify elemental antimony as the majorphase in the coating with some magnetite (Fe₂ O₃) and FeSb₂.

The foregoing examples are illustrative of the present invention and arenot to be construed as limiting thereof.

That which is claimed is:
 1. An absorption solution for refrigerationsystems, consisting essentially of at least one alkali metal halide inan amount of at least about 20 percent by weight based on the totalweight of the solution, at least one bromide of a Group Va metallicelement in an amount sufficient to provide a corrosion inhibitingeffect, optionally zinc halide, and optionally lithium nitrate.
 2. Thesolution of claim 1, wherein said at least one bromide of a Group Vametallic element comprises antimony bromide (SbBr₃).
 3. The solution ofclaim 1, wherein said at least one Group Va metallic element is presentas bromide in an amount of about 150 ppm to about 400 ppm.
 4. Thesolution of claim 1, wherein said at least one alkali metal halide isselected from the group consisting of lithium bromide, lithium chloride,lithium iodide and mixtures thereof.
 5. The solution of claim 1, whereinsaid alkali metal halide is present in an amount of about 40 to about 65percent by weight based on the total weight of the solution.
 6. Thesolution of claim 1, wherein said at least one bromide of a Group Vametallic element is selected from the group consisting of antimonybromide, arsenic bromide, bismuth bromide, and mixtures thereof.
 7. Thesolution of claim 1, wherein said bromide is present in an amount fromabout 500 ppm to about 5000 ppm.
 8. The solution of claim 1, whereinsaid bromide is present in an amount from about 1000 ppm to about 5000ppm.
 9. An absorption solution for refrigeration systems, consistingessentially of at least one lithium halide in an amount of at leastabout 20 percent by weight based on the total weight of the solution andantimony bromide (SbBr₃).
 10. The solution of claim 9, wherein saidalkali metal halide is present in an amount of about 40 to about 65percent by weight based on the total weight of the solutions.
 11. Thesolution of claim 9, wherein said bromide is present in an amount fromabout 500 ppm to about 5000 ppm.
 12. The solution of claim 9, whereinsaid bromide is present in an amount from about 1000 ppm to about 5000ppm.
 13. A process for inhibiting the corrosion of a refrigerationmachine resulting from the presence of alkali metal halide absorbentsolutions, comprising circulating in a refrigeration machine anabsorption solution consisting essentially of at least one alkali metalhalide in an amount of at least about 20 percent by weight based on thetotal weight of the solution, at least one bromide of a Group Vametallic element in an amount sufficient to provide a corrosioninhibiting effect, optionally zinc halide and optionally lithiumnitrate.
 14. The process of claim 13 , wherein said at least one bromideof a Group Va metallic element is antimony bromide (SbBr₃).
 15. Theprocess of claim 13, wherein said at least one Group Va metallic elementis present as bromide in an amount of about 150 ppm to about 400 ppm.16. The process of claim 13, wherein said at least one alkali metalhalide is selected from the group consisting of lithium bromide, lithiumchloride, lithium iodide and mixtures thereof.
 17. The process of claim13 wherein during said circulating step, said at least one bromide of aGroup Va metallic element forms a protective layer on a surface withinsaid machine.
 18. The process of claim 13, wherein during saidcirculating step said solution is exposed to temperatures ranging fromabout 150° F. to about 550° F.
 19. The process of claim 13, wherein saidalkali metal halide is present in an amount of about 40 to about 65percent by weight based on the total weight of the solution.
 20. Theprocess of claim 13, wherein said at least one bromide of a Group Vametallic element is selected from the group consisting of antimonybromide, arsenic bromide, bismuth bromide and mixtures thereof.
 21. Theprocess of claim 13, wherein said bromide is present in an amount fromabout 500 ppm to about 5000 ppm.
 22. The process of claim 13, whereinsaid bromide is present in an amount from about 1000 ppm to about 5000ppm.